Benzo[a]pyrene [B(a)P] is one of the polycyclic aromatic hydrocarbons compounds that is formed during incomplete combustion of coal, gas, wood, and organic substances. This compound is a ubiquitous environmental pollutant, which is mutagenic and carcinogenic to both humans and animals. Many metabolites are formed dining the metabolism of B(a)P but (+/-)-benzo[a]pyrene-7,8-diol-9,10-epoxide is the most highly reactive one that binds with DNA and forms adducts at the N-7 guanine residue eventually leading to cellular transformation. Selenium may play a role in slowing this process of cellular transformation in vitro. Selenium has two major characteristics in that it is a mandatory component for the glutathione peroxidase scavenging process and also a necessary trace element and may potential be used as an adjunct treatment of many cancers such as breast cancer. Preliminary data from our lab suggest that selenium in moderate concentration (12.5 ppm) has an inhibitory effect on the growth of MDA-MB-435 breast cancer cells. The objective of this project is to chazactetize the anti-carcinogenic properties of selenium, by evaluating its effects on B(a)P treated cells. Therefore, the central hypothesis is that prior selenium treatment of 3T3 and CHO cells decreases the potential via the glutathione peroxidase pathway, to transform into malignant form subsequent to B(a)P exposure.